[bitcoin-dev] Hardfork bit BIP

Jorge Timón jtimon at jtimon.cc
Fri Feb 5 09:58:36 UTC 2016

Concept ACK. I've been talking about adding this to BIP99 since before
scaling bitcoin Hong Kong, so it will be nice to have a BIP to just point
to. Also I hadn't thought about concurrent deployment of 2 hardforks, nice.

On Feb 4, 2016 23:30, "Gavin Andresen via bitcoin-dev" <
bitcoin-dev at lists.linuxfoundation.org> wrote:

> If the worry is full nodes that are not upgraded, then a block with a
negative version number will, indeed, fork them off the the chain, in
exactly the same way a block with new hard-forking consensus rules would.
And with the same consequences (if there is any hashpower not paying
attention, then a worthless minority chain might continue on with the old

Additionally, a warning or special error could be thrown when a block is
rejected due to the hardfork bit being activated.

> I think a much better idea than this proposed BIP would be a BIP that
recommends that SPV clients to pay attention to block version numbers in
the headers that they download, and warn if there is a soft OR hard fork
that they don't know about.

Although I agree this PR should include such warning/error recommendations,
SPV nodes can't tell whether a change is a hardfork or a softfork just by
looking at the version bits, even in the case of uncontroversial hardforks
deployed with bip9 as recommended by bip99. For controversial hardforks
where bip9 should NOT be used for deployment, setting the hardfork bit is
even more important.

> It is also a very good idea for SPV clients to pay attention to
timestamps in the block headers that the receive, and to warn if blocks
were generated either much slower or faster than statistically likely.
Doing that (as Bitcoin Core already does) will mitigate Sybil attacks in

This seems out of the scope of this PR.
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